Organic light emitting devices (OLEDs) are thin-film light emitting devices made from organic semiconductive materials and driven by direct current voltage.
Simply speaking, the light emitting mechanism of OLEDs is as follows: electrons and holes, driven by a certain voltage, are injected into the electron transporting layer and the hole transporting layer, respectively, from the cathode and the anode; electrons and holes meet each other to form excitons (generally classified as singlet excitons and triplet excitons), which excites the luminescent molecules in the luminescent material to emit visible lights.
Conventional organic fluorescent materials can utilize only 25% of electrically excited singlet excitons to emit light, leading to low internal quantum efficiency of the device (25% at most). Phosphorescent material has enhanced intersystem crossing due to strong spin-orbit coupling of heavy atom center, and can effectively utilize singlet excitons and triplet excitons formed by electrical excitation to emit light, and theoretically may make the device to reach an internal quantum efficiency of 100%. However, phosphorescent materials contain heavy metals, and have disadvantages such as high costs, low stability of the material, low efficacy of the device, which limit the applications thereof in OLEDs.
Thermally activated delayed fluorescence (TADF, also known as E-type Delayed Fluorescence) materials are the third generation of organic luminescent materials developed after organic fluorescent materials and organic phosphorescent materials.
TADF materials normally have small singlet-triplet energy level difference (ΔEST), so that triplet excitons can be converted to singlet excitons through reverse intersystem crossing (RISC) to emit light. Therefore, TADF materials can sufficiently utilize singlet excitons and triplet excitons formed by electrical excitation to emit light, and theoretically may also make the device to reach an internal quantum efficiency of 100%. In addition, due to RISC, the lifetime of the light produced by this type of materials is longer than that of traditional fluorescence or phosphorescence. Furthermore, TADF materials have controllable structures, stable properties, low prices, and are free of precious metals, and therefore have broad prospect of being used in the field of OLEDs.
The current studies on TADF materials are focused on how to lower the singlet-triplet energy level difference ΔEST to a value which meets the requirement of RISC. Theoretically, when ΔEST≤0.2 eV, RISC can be realized.
Upon research, it has been found that there is a positive correlation between ΔEST and the degree of orbital overlap between HOMO and LUMO, wherein HOMO refers to highest occupied molecular orbital, and LUMO refers to lowest unoccupied molecular orbital. If ΔEST needs to be lowered, this may be achieved by separating HOMO from LUMO as much as possible while ensuring the recombination of excitons.
Currently, there lacks research on the chemical structures, optical properties and physical properties of TADF materials and the correlation thereof with OLEDs, which limits the development of new TADF materials, leading to lacking of variety of current TADF materials, which cannot meet the current requirements on the development of OLEDs.